Instruments for gas detection are widely used to protect workers in potentially hazardous environments. Gaseous hazards may be due to the presence of flammable gases, toxic gases or oxygen concentrations higher or lower than normal air (20.9% v/v). Typical environments where hazardous atmospheres are encountered include oil production and petrochemical industries, steel and paper mills, sewers, water treatment plants, ship holds etc. Whenever workers need to be in a potentially hazardous atmosphere, they must have protection. Gas detection instruments provide this protection by continuously monitoring the atmosphere and if a potentially dangerous conditions exits, the gas detection instrument activates an alarm (typically visual, audible or vibration) to provide a warning of the hazard to the user.
Gas detection instruments typically contain one or more sensors that detect the potentially hazardous gases. There are many types of sensors, including electrochemical, optical, semiconductor, heat of combustion (e.g. catalytic bead) etc., but they all provide an electrical output that varies with the gas concentration. The technology of sensors is well known in the prior art; for example details of several commonly used sensors can be found in xe2x80x9cTechniques and Mechanisms in Gas Sensingxe2x80x9d, Ed. P. T. Moseley, J. O. W. Norris, D. E. Williams, Publ. Adam Hilger, Bristol, 1991. ISBN 0-7503-0074-4.
Most sensors give a relative output, with, for example, the output current from an amperometric gas sensor being proportional to the gas concentration. While it may be possible to calculate the proportionality constant in principle (see for example P. R. Warburton et al., Analytical Chemistry (1998), 70, 998-1006), in practice the proportionality is almost always found by calibration. In the calibration process, a test gas of known concentration is applied to the gas detection instrument and the instrument reading is then adjusted to match the concentration of the test gas.
The calibration process is also repeated periodically with most gas detection instrumentation since the output of sensors can vary with time and conditions. The frequency of calibration depends on the stability of the sensor output with respect to time, the accuracy of the reading required and the gravity of the consequences if the reading is inaccurate. Since gas detection instruments for workplace safety are used to protect people""s lives, these instruments are usually calibrated more frequently than instruments used in many other applications.
Gas detection instruments may be portable and in many cases they may be small enough to be worn. This type of use provides so called personal protection, since the instrument protects the individual worker. Another common mode of use is to mount the instrument to a wall or other object. These so called fixed instruments provide area monitoring.
In most cases, gas enters the instrument by diffusion. However, in some applications, the sample gas is pumped into the instrument by means of an internal or external pump.
The typical calibration frequency for an instrument using electrochemical or heat of combustion sensors is monthly. In calibration, it is important to ensure that the test gas is delivered to the sensor in a consistent manner from calibration to calibration. Furthermore, since the response of many types of sensors depends on the gas flow rate, it is important to ensure that the response of the sensor during calibration matches that during use. Thus for example, if a diffusion mode instrument is calibrated to 100 ppm carbon monoxide, then this instrument should read 100 ppm when placed in an atmosphere of 100 ppm carbon monoxide.
In order to ensure a repeatable and desired gas delivery to the instrument, calibration often involves use of a calibration adapter. A calibration adapter is a device that is usually attached to the instrument and covers or seals the gas path to the sensors. A regulated flow of gas, such as from a compressed gas cylinder is supplied to the calibration adapter. The calibration adapter performs two functions: it ensures that the gas delivery to the instrument during calibration is repeatable from calibration to calibration in any environment, and ensures that the sensor reading from the gas flow under calibration conditions matches the reading of the instrument when exposed to the same concentration of gas under normal use conditions.
Most manufacturers of gas detection instruments use calibration adapters. The calibration adapter is usually designed for a specific instrument or for a group of similar instruments and parts are usually not interchangeable with other instruments. Further details of gas detection instruments and calibration can be found in standard texts such as S. A. Ness xe2x80x9cAir Monitoring For Toxic Exposuresxe2x80x9d, Van Nostrand Reinhold, N.Y. (1991) and B. S. Cohen, S. V. Hering (Ed.), xe2x80x9cAir Sampling Instruments for Evaluation of Atmospheric Contaminantsxe2x80x9d, 8th Edition, ACGIH, Cincinnati, Ohio (1995). Since calibration adapters are usually only used for calibration, they are often mislaid or lost and are no longer readily available for calibration. While new calibration adapters are available from the instrument manufacturers, time and resources can be wasted obtaining a calibration adapter and a needed calibration delayed.
It is therefore an object of the invention to provide a calibration adapter which is part of the gas detection instrument, ensuring that the calibration adaptor is always available for calibrating the instrument.
It is a further object of the invention to provide a calibration adapter which is mechanically attached to the instrument in such a way that it cannot be removed during normal use of the instrument.
To achieve these and other objects, the invention is directed to a calibration adaptor for a gas sensing device having a gas entry port therein, comprising:
a base piece having a gas entry port therein;
means for fixedly attaching the base piece to a surface of the gas sensing device, and allowing movement of the base piece between a first position in which the gas entry port of the gas sensing device is open to detect ambient gases, and a second position in which the gas entry port of the base piece is aligned with the gas entry port of the gas sensing device and controls entry of gas into the gas entry port of the gas sensing device;
means for releasably retaining the base piece in the first position;
means for releasably retaining the base piece in the second position; and
means for attaching a calibration gas source to the gas entry port of the base piece.
In preferred embodiments of the invention, the calibration adapter can be in the form of a flap that is hinged or in the form of a slide. The calibration adaptor can then be in two positions, 1) normal operation and 2) calibration.
The attached calibration adapter can have other functions, such as forming a sound chamber for the audio alarm, and it can increase the alarm sound level.